NASA astronauts who will be riding the SpaceX Dragon discussed the challenges they’ve faced and what they’re looking forward to while speaking at the SpaceX headquarters in Hawthorne, California on Aug. 13 ahead of the Crew Dragon mission.

NASA last week assigned the astronauts who will ride the first commercial capsules into orbit next year and bring crew launches back to the U.S.

“Being able to fly a first flight of a vehicle as a test pilot is a once-in-a-generational type of opportunity,” said astronaut Doug Hurley, who will be on the first crew of the SpaceX Dragon.

SpaceX and Boeing are shooting for a test flight of their capsules by the end of this year or early next, with the first crews flying from Cape Canaveral, Florida, by next spring or summer.

Nine astronauts were named to ride the SpaceX Dragon and Boeing Starliner capsules—five on the first crew flights and four on the second round of missions to the International Space Station.

NASA has been paying billions of dollars to SpaceX and Boeing to develop the crew capsules to pick up where the shuttles left off, while also paying billions for cargo deliveries to the space station by SpaceX and Northrop Grumman. The cargo missions started in 2012. The crew missions have been delayed repeatedly because of the technical challenges and difficulties of making spacecraft safe for humans. A recent abort test by Boeing resulted in leaking engine fuel.